1. Field of the Invention
The present invention relates to loss-of-light detecting apparatuses and, more particularly, to a loss-of-light detecting apparatus detecting any loss of light.
2. Description of the Related Art
A basic architecture for optical transmission called Optical Transport Network (OTN) according to International Telecommunication Union Telecommunication Standardization sector (ITU-T) recommendations is used as an optical core network and development of the OTN has been advanced in recent years. In the OTN based on Wavelength Division Multiplexing (WDM) optical transmission, not only signals in telephone services but also signals in Internet Protocol (IP) or Ethernet® services can be processed.
FIG. 16 is a block diagram showing the configuration of a typical WDM apparatus. The WDM apparatus 50 includes a transponder unit 51 and a WDM unit 52 at the transmission side and includes a WDM unit 53 and a transponder unit 54 at the reception side.
The transponder unit 51 includes transponders 51-1 to 51-n corresponding to n-number channels. The WDM unit 52 includes an optical multiplexer 52a and a WDM post-amplifier 52b. The WDM unit 53 includes an optical demultiplexer 53a and a WDM pre-amplifier 53b. The transponder unit 54 includes transponders 54-1 to 54-n corresponding to n-number channels. Typically, arrayed waveguide gratings (AWGs) are used in the optical multiplexer 52a and the optical demultiplexer 53a, and Erbium Doped Fiber Amplifiers (EDFAs) are used as the amplifiers.
The transponders 51-1 to 51-n each receive an optical signal transmitted from a client and convert the optical signals into signals within a waveband appropriate for the WDM (wideband to narrowband conversion of the wavelengths). The optical multiplexer 52a multiplexes the signals of the multiple wavelengths resulting from the wavelength conversion to generate a WDM signal. The WDM post-amplifier 52b amplifies the WDM signal and supplies the amplified WDM signal to a network 5 through an optical fiber transmission path F.
The WDM pre-amplifier 53b receives the WDM signal transmitted from the network 5 through the optical fiber transmission path F and amplifies the received WDM signal, The optical demultiplexer 53a performs wavelength filtering to demultiplex the amplified WDM signal into an optical signal of each wavelength and outputs the optical signals. The transponders 54-1 to 54-n each receive the optical signal of each wavelength, convert the received optical signals into the signals of the original wavelengths (narrowband to wideband conversion of the wavelengths), and transmit the signals to the client.
The transponders 54-1 to 54-n each includes an optical amplifier and a dispersion compensator (not shown). The optical signal demultiplexed by the optical demultiplexer 53a is amplified by the optical amplifier in each of the transponders 54-1 to 54-n. The dispersion compensator in each of the transponders 54-1 to 54-n compensates for wavelength dispersion caused on the optical fiber transmission path F. The wavelength conversion is performed after the dispersion compensation.
In addition, the transponders 54-1 to 54-n each detect any loss of light on the basis of the power of the input optical signal. If any loss of light is caused, the transponders 54-1 to 54-n each perform shutdown control of the optical amplifier. For example, when the transponders 54-1 to 54-n use variable dispersion compensators, the transponders 54-1 to 54-n stop the dispersion compensation operation.
FIG. 17 illustrates an exemplary spectrum of optical signals. Referring to FIG. 17, the wavelength increases from left to right. An exemplary spectrum of an optical signal that passes through the optical fiber transmission path F and that is demultiplexed by the optical demultiplexer 53a and is supplied to the transponder unit 54 is illustrated in FIG. 17. The level of the optical signal flowing through the optical fiber transmission path F is raised by amplified spontaneous emission (ASE) light. The ASE light typically has wavelength dependence. As shown in FIG. 17, the level of the ASE light is not constant in the waveform amplification area (1,560 to 1,630 nm in the lower band) of the EDFA and continuously decreases rightward, that is, continuously decreases toward the longer wavelengths.
Accordingly, even if the level of the optical signals transmitted from the transponders 51-1 to 51-n is constant, the power of the ASE light in the optical signals amplified by the EDFAs on the transmission path is not constant for different wavelengths. Consequently, the power of the optical signals supplied to the transponders 54-1 to 54-n through the optical demultiplexer 53a is not constant for different wavelengths even if the power of the main signal component in the optical signal of each channel is constant.
FIG. 1B is a conceptual diagram of the reception capacity of a transponder. The reception capacity of a transponder is determined by the power of the input optical signal (the input light power) and the optical signal-to-noise ratio (OSNR). The OSNR is a ratio between the level of the optical signal and the level of the ASE light.
Since a channel CH2 has a sufficient input light power and the OSNR reference value of the channel CH2 is met (the level of the main signal component (data component) is higher than that of the ASE light component to an extent where the OSNR reference value is met), it is possible for the transponder to recognize data through the channel CH2. Since a channel CH1 has a sufficient input light power but the OSNR reference value of the channel CH1 is not met because the level of the ASE noise component is considerably higher than that of the main signal component, it is not possible for the transponder to recognize data through the channel CH1. Since a channel CH3 has an input light power lower than those of the channels CH2 and CH1 but the OSNR reference value of the channel CH3 is met, it is possible for the transponder to recognize data through the channel CH3.
The OSNR is determined by, for example, the frequency of the optical signal, the modulation method adopted in the apparatus, and the reception performance of the apparatus. When an Out of Band-Forward Error Correction (OOB-FEC) function is used to perform non return-to-zero modulation to a 10-Gbps optical signal, the OSNR is normally equal to about 15 dB. If the transmission bit rate is increased from 10 Gbps to 40 Gbps, the OSNR reference value is increased by about 6 dB.
The transponder can receive the optical signal if the input optical signal meets the conditions of the input light power and the OSNR, as described above. However, since it is determined whether the optical signal is received only on the basis of the input light power in the loss of light detecting function of transponders in related art, the transponders cannot detect any loss of light even if the loss of light is caused as in the channel CH1 and possibly erroneously recognize that the optical signal is normally received.